Nitrogen-sulfur-carbon nanocomposites and their application as cathode materials in lithium-sulfur batteries

ABSTRACT

The invention is directed in a first aspect to electron-conducting porous compositions comprising an organic polymer matrix doped with nitrogen atoms and having elemental sulfur dispersed therein, particularly such compositions having an ordered framework structure. The invention is also directed to composites of such S/N-doped electron-conducting porous aromatic framework (PAF) compositions, or composites of an S/N-doped mesoporous carbon composition, which includes the S/N-doped composition in admixture with a binder, and optionally, conductive carbon. The invention is further directed to cathodes for a lithium-sulfur battery in which such composites are incorporated.

The present application is a continuation of U.S. application Ser. No.15/248,349 filed Aug. 26, 2016 which is a continuation of U.S.application Ser. No. 13/683,520 filed Nov. 21, 2012, the contents ofwhich are incorporated herein by reference in their entirety.

This invention was made with government support under Prime Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to cathode materials for lithiumion batteries, and more particularly, cathode materials forlithium-sulfur batteries.

BACKGROUND OF THE INVENTION

Lithium-ion batteries have found widespread usage as electrical energystorage devices in various portable electronics because of their lightweight relative to other types of batteries. However, particularly forhigh power applications such as electric vehicles, there has been acontinuing effort to improve the energy output and useful lifetime inlithium ion batteries to better suit these high power applications.

Lithium-sulfur (Li/S) batteries, in particular, hold great promise forhigh power applications. Lithium-sulfur batteries have a theoreticalcapacity of 1675 mAhg⁻¹, nearly one magnitude higher than that ofLiFePO₄ (theoretical capacity of 176 mAhg⁻¹). Nevertheless, the Li/Ssystem has not yet been implemented in high power applications becauseof two significant obstacles: the poor electrical conductivity ofelemental sulfur and the intrinsic polysulfide shuttle, both of whichcontribute to capacity fade with cycling.

The electrical conductivity of elemental sulfur is as low as 5×10⁻³⁰S/cm at 25° C. Such a low conductivity causes poor electrochemicalcontact of the sulfur and leads to low utilization of active materialsin the cathode. Although compositing elemental sulfur with carbon orconducting polymers significantly improves the electrical conductivityof sulfur-containing cathodes, the porous structure of the cathode stillneeds optimization to facilitate the transport of ions while retainingthe integrity of the cathode after dissolution of sulfur at thedischarge cycle.

The sulfur in the cathode, except at the full charge state, is generallypresent as a solution of polysulfides in the electrolyte. Theconcentration of polysulfide species S_(n) ²⁻ with n greater than 4 atthe cathode is generally higher than that at the anode, and theconcentration of S_(n) ²⁻ with n smaller than 4 is generally higher atthe anode than the cathode. The concentration gradients of thepolysulfide species drive the intrinsic polysulfide shuttle between theelectrodes, and this leads to poor cyclability, high current leakage,and low charge-discharge efficiency.

Most importantly, a portion of the polysulfide is transformed intolithium sulfide, which is deposited on the anode. This depositionprocess occurs in each charge/discharge cycle and eventually leads tothe complete loss of capacity of the sulfur cathode. The deposition oflithium sulfide also leads to an increase of internal cell resistancedue to the insulating nature of lithium sulfide. Progressive increasesin charging voltage and decreases in discharge voltage are commonphenomena in lithium-sulfur batteries because of the increase of cellresistance in consecutive cycles. Hence, the energy efficiency decreaseswith the increase of cycle numbers.

Much research has been conducted to mitigate the negative effect of thepolysulfide shuttle. The bulk of this research has focused on either theprotection of lithium anode or the restraining of the ionic mobility ofthe polysulfide anions. However, protection of the lithium anode leadsto the passivation of the anode, and this in turn causes a slow reactionrate of the anode during the discharge cycle. Therefore, protection ofthe lithium anode leads to the loss of power density. Gel electrolytesand solid electrolytes have also been used as a means for slowing downthe polysulfide shuttle by reducing the ionic mobility of electrolytes.However, the slow transport of ions leads to a low power density.Moreover, neither the protection of lithium anode nor the restraining ofionic mobility completely shuts down the polysulfide shuttle. Althoughthe polysulfide shuttle occurs at slow speed, such modified Li/Sbatteries generally suffer from a significantly shortened lifespan ascompared to lithium ion batteries without these modifications.

Accordingly, there is a need for lithium-sulfur batteries with improvedperformance, particularly with respect to initial discharge capacities,cycling performance, rate capability, and electrical power output (i.e.,improved power density), as well as improved usable lifetime. Therewould be a particular benefit in a lithium-sulfur battery possessingboth an improved power output and an improved usable lifetime. Inachieving the aforementioned goals, there is a particular need for alithium-sulfur battery design that minimizes or altogether prevents theirreversible deposition of lithium sulfide on the lithium anode of thebattery.

SUMMARY OF THE INVENTION

In one aspect, the invention is directed to new composite materialsuseful as cathodic materials for a lithium-sulfur battery. Specialdesign features have been incorporated into the composite material thatpermits the composite material to substantially minimize the formationof lithium sulfide at the anode and to improve initial dischargecapacities, cycling performance, rate capabilities, and usable lifetime.

In a first set of embodiments, the composite material includes a novelelectron-conducting porous composition composed of an organic polymermatrix doped with nitrogen atoms and having elemental sulfur dispersedtherein. For example, the electron-conducting porous composition can becomposed of an ordered framework structure in which nitrogen atoms areinterconnected by unsaturated hydrocarbon linkers, wherein the orderedframework structure contains micropores in which sulfur is incorporated.In a second set of embodiments, the composite material includes amesoporous carbon composition doped with nitrogen atoms and havingelemental sulfur dispersed therein.

The composite typically also includes an amount of conductive carbon(e.g., 10-30 wt % by weight of the composite) and a binder, such asPVDF. In particular embodiments, the conductive carbon is carbon black.In other particular embodiments, the conductive carbon is or includescarbon nanotubes.

In another aspect, the invention is directed to a lithium-sulfur batterycontaining a cathode that contains any of the abovenitrogen-sulfur-carbon composite materials. The lithium-sulfur batterycan employ a liquid, solid, or gel electrolyte that includes a lithiumsalt. In particular embodiments, the lithium-sulfur battery employs anionic liquid electrolyte. The ionic liquid can be, for example, apyrrolidinium or piperidinium ionic liquid.

In other aspects, the invention is directed to a method of operating alithium-sulfur battery that includes any of the cathode compositematerials and/or electrolytes (particularly ionic liquids) describedherein. The invention is also directed to methods of preparing thecathode composite materials as well as methods for assembling lithiumsulfur batteries that include these cathode composite materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Graph showing thermogravimetric analysis (TGA) trace for PAF(porous aromatic framework) and its sulfur-doped version PAF-S

FIGS. 2A, 2B. Graphs showing charge/discharge profile (FIG. 2A) andcycle performance (FIG. 2B) of Li—S batteries based on PAF-S compositeelectrodes in 1.0 M LiPF₆/MiPS at a current density of 84 mA g⁻¹ (20 wt% carbon) at room temperature.

FIGS. 3A, 3B. Graphs showing charge/discharge profile (FIG. 3A) andcycle performance (FIG. 3B) of Li—S batteries based on PAF-S compositeelectrodes in 0.5 M LiTFSI/MPPY.TFSI at a current density of 84 mA g⁻¹(20 wt % carbon) at 50° C.

FIG. 4. Graph showing TGA trace for S/N-doped mesoporous carbonmaterial.

FIGS. 5A, 5B. Graphs showing charge/discharge profile (FIG. 5A) andcycle performance (FIG. 5B) of Li—S batteries based on S/N-dopedmesoporous carbon composite electrodes in 0.5 M LiTFSI/MPPY.TFSI at acurrent density of 84 mA g⁻¹ (20 wt % carbon) at 50° C.

FIG. 6. Graph showing TGA trace for S/N-doped mesoporous carbon materialadmixed with carbon nanotube (“CNT-mesoporous carbon”).

FIGS. 7A, 7B. Graphs showing charge/discharge profile (FIG. 7A) andcycle performance (FIG. 7B) of Li—S batteries based on CNT-mesoporouscarbon composite electrodes in 1.0 M LiPF₆/MiPS at a current density of84 mA g⁻¹ (20 wt % carbon) at room temperature.

FIGS. 8A, 8B. Graphs showing charge/discharge profile (FIG. 8A) andcycle performance (FIG. 8B) of Li—S batteries based on CNT-mesoporouscarbon composite electrodes in 0.5 M LiTFSI/MPPY.TFSI at a currentdensity of 84 mA g⁻¹ (20 wt % carbon) at room temperature.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “about” generally indicates within ±0.5%, 1%,2%, 5%, or up to ±10% of the indicated value. For example, a pore sizeof about 10 nm generally indicates in its broadest sense 10 nm±10%,which indicates 9.0-11.0 nm. In addition, the term “about” can indicateeither a measurement error (i.e., by limitations in the measurementmethod), or alternatively, a variation or average in a physicalcharacteristic of a group (e.g., a population of pores).

In a first aspect, the invention is directed to an electron-conducting(i.e., conductive) porous composition containing an organic polymermatrix doped with nitrogen atoms and having elemental sulfur disposedtherein. The term “electron-conducting” or “conductive”, as used herein,includes any degree of conductivity, including semiconducting,conducting, or superconducting. The term “organic”, as used herein, isdefined as a composition that includes carbon atoms, and more typically,carbon atoms engaged in carbon-carbon single, double, and/or triplebonds. The organic polymer matrix is typically unsaturated (i.e., bycontaining carbon-carbon double or triple bonds) in order to permitelectron conduction. Generally, the unsaturated bonds are conjugated(i.e., delocalized) to permit electron conduction. In particularembodiments, the nitrogen-doped conductive polymer includes aromaticgroups (for example, phenyl) and/or aromatic linkages (e.g., phenylene).The nitrogen-doped organic polymer matrix can be, for example, apolypyrrole, polyaniline, polyimine (e.g., aromatic polyimine), ornitrogen-doped poly(p-phenylene vinylene) conductive polymer.

In particular embodiments, the electron-conducting porous composition isor includes an ordered framework structure in which nitrogen atoms areinterconnected by unsaturated hydrocarbon linkers. The hydrocarbonlinkers can be any hydrocarbon linker known in the art, typicallyunsaturated, such as vinylene, o-, m-, or p-phenylene, p-biphenylene,p-terphenylene, 4,4′-stilbene, and 1,5-naphthalene. The term “frameworkstructure”, as used herein, denotes a structural order either in one,two, or three dimensions, analogous in some respects to the order foundin metal-organic frameworks (MOFs). The framework structure may havecrystalline attributes or may be altogether crystalline orsemi-crystalline. In other embodiments, the framework is non-crystalline(i.e., amorphous) but has an ordered pore size or ordered pore sizedistribution. The ordered nature of the framework material preferablypossesses a substantially uniform pore size distribution.

The electron-conducting porous composition can have any suitable poresize and pore size distribution. The pores can be, for example,micropores, mesopores, or macropores, or a combination thereof. The term“micropores”, as used herein and as commonly understood in the art,refer to pores having a size of less than 2 nm. In differentembodiments, the micropores can have a size of precisely, about, atleast, up to, or less than, for example, 0.5, 0.8, 1, 1.2, 1.5, 1.8, or2 nm, or a pore size within a range bounded by any two of the foregoingexemplary values. The term “mesopores”, as used herein and as commonlyunderstood in the art, refers to pore sizes of 2 to 50 nm. In differentembodiments, the mesopores can have a size of precisely, about, atleast, above, up to, or less than, for example, 2, 3, 4, 5, 8, 10, 12,15, 18, 20, 25, 30, 35, 40, 45, or 50 nm, or a pore size within a rangebounded by any two of the foregoing exemplary values. The term“macropores”, as used herein and as commonly understood in the art,refers to pore sizes above 50 nm. In different embodiments, themacropores can have a size of precisely, about, at least, above, up to,or less than, for example, 50, 60, 70, 80, 90, 100, 150, 200, or 250 nm,or a pore size within a range bounded by any two of the foregoingexemplary values. In some embodiments, the electron-conducting porouscomposition possesses only micropores, or only mesopores, or onlymacropores. In other embodiments, the electron-conducting porouscomposition possesses a proportion of pores as micropores and mesopores,or micropores and macropores, or mesopores and macropores, ormicropores, mesopores, and macropores. Generally, micropores andmesopores have a circular shape, which can be an approximately circular(e.g., ellipsoidal) or completely circular shape. For pores having acircular shape, the pore size refers either to the surface diameter ofthe pore (in the case of a completely circular pore) or the longestsurface diameter of the pore (in the case of an elliptical pore). Thepores can also be non-circular, or even irregular-shaped.

The pores can also possess a degree of uniformity. The uniformity can bein any desired property, such as the pore diameter (pore size), wallthickness, or inter-pore spacing. Typically, by being substantiallyuniform is meant that the pores show no more than 15% or 10%, and morepreferably, no more than 5%, 2%, 1%, 0.5%, or 0.1% deviation (i.e.,absolute or standard deviation) in one or more attributes of the pores,particularly pore size. In a particular embodiment, the pores possess anordered spatial arrangement with each other. In some embodiments, theordered arrangement includes a patterned or symmetrical spatialarrangement of the pores. The patterned spatial arrangement can be, forexample, a hexagonal close packed or cubic arrangement. The pore sizedistribution may also be characterized as monomodal, bimodal, trimodal,or a higher modality.

The electron-conducting porous composition can have any suitable totalpore volume. For example, in different embodiments, the total porevolume can be at least 0.5, 1, 1.5, 2, 2.2, or 2.5 cm³/g, or a porevolume within a range bounded by any two of these values.

The electron-conducting porous composition can have any suitable wallthickness of the pores. For example, in different embodiments, the wallthickness can be about, at least, or less than 1 nm, 2 nm, 3 nm, 4 nm, 5nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 15 nm, 18 nm, 20 nm, 25nm, or 30 nm, or within a range bounded by any two of these values.

The electron-conducting porous composition can also have any suitablesurface area. For example, in different embodiments, the surface areacan be at least 300 m²/g, 400 m²/g, 500 m²/g, 600 m²/g, 700 m²/g, 800m²/g, 900 m²/g, 1000 m²/g, 1200 m²/g, 1500 m²/g, 1800 m²/g, 2000 m²/g,2500 m²/g, 3000 m²/g, 3500 m²/g, 4000 m²/g, 4500 m²/g, or 5000 m²/g, ora surface area within a range bounded by any two of these values.

In the electron-conducting porous composition, sulfur is incorporatedeither within the matrix, the pores, or both. The sulfur can be in theform of, for example, elemental sulfur (S), sulfide (S²⁻), or apolysulfide (S_(n) ²⁻, wherein n is a number greater than 1, and up to,for example, 2, 3, 4, 5, 6, 7, 8, 9, or 10). At least a portion of thepores is occupied by (i.e., “filled with” or “contains”) elementalsulfur. The portion of pores occupied by elemental sulfur can be about,at least, above, up to, or less than, for example, 1, 2, 5, 10, 20, 30,40, 50, 60, 70, 80, 90, 95, 98, 99, or 100 volume % (vol %) of thepores.

The amount of sulfur contained in the sulfur-carbon composite (i.e., the“sulfur loading” in terms of weight percentage (wt %) of sulfur by totalweight of the sulfur-carbon composite) generally depends on the totalpore volume of the composite material. Accordingly, the sulfur loadingcan be adjusted by suitable adjustment of the total pore volume. As thetotal pore volume increases, higher sulfur loadings are made possible.Thus, by suitable adjustment of the pore volume, a sulfur loading ofabout, at least, above, up to, or less than, for example, 5, 10, 15, 20,25, 30, 35, 40, 45, or 50 wt % by weight of the electron-conductingporous composition can be attained.

In some embodiments, the electron-conducting porous composition furtherincludes one or more dopants that facilitate or cause the composition tobe conducting or semi-conducting. Some examples of such dopants includeacids (i.e., inorganic or organic acids), bases (inorganic or organicbases), and oxidants, such as halogens (e.g., chlorine, bromine, oriodine). In some embodiments, one or more (or all) of such dopants areexcluded.

In particular embodiments, the electron-conducting porous compositioncontains interconnected triphenylamine units. A particular example ofsuch an electron-conducting porous composition (i.e., PAF) is describedin T. Ben, et al., J. Mater. Chem., 21, 18208 (2011), the disclosure ofwhich is herein incorporated by reference in its entirety. PAF ismicroporous with a uniform micropore distribution centered at about 1.2nm.

In other particular embodiments, the electron-conducting porouscomposition is a mesoporous carbon composition, such as any of themesoporous carbon compositions known in the art, that has been modifiedto include nitrogen and sulfur. The mesoporous carbon composition beforesuch modification can be, for example, as described in C. Liang, et al.,J. Am. Chem. Soc., 128, pp. 5316-5317 (2006) or C. Liang, et al., Chem.Mater., 21, pp. 4724-4730 (2009), the disclosures of which are hereinincorporated by reference in their entirety.

The electron-conducting porous composition can be prepared by anysuitable method. In one set of embodiments, the electron-conductingporous composition is prepared by treating an N-doped porous carbontemplate with sulfur under elevated temperature conditions effective forincorporating the sulfur into the carbon matrix. In other embodiments,the electron-conducting porous composition is prepared by treating aporous carbon template with both nitrogen (e.g., ammonia) and elementalsulfur, typically as separate steps, in a process where nitrogen andsulfur become incorporated into the carbon matrix. In yet otherembodiments, the electron-conducting porous composition is prepared bycarbonizing a precursor that contains a carbonizable matrix, nitrogen,and sulfur. Volatile agents can be included in the precursor to producea porous structure during carbonization.

In some embodiments, the electron-conducting porous composition is notcarbonized, and thus, does not have a carbon matrix; instead, thecomposition contains organic linking groups to nitrogen with sulfurincorporated into pores therein. For example, in one set of embodiments,an electron-conducting N-doped porous framework composition, such asPAF, described above, is heated with sulfur, below a carbonizationtemperature, in order to incorporate sulfur therein.

In some embodiments, the method includes impregnating a porousnitrogen-doped carbon component, having the characteristics describedabove, and as prepared by methods known in the art, with a solution ofelemental sulfur. The elemental sulfur considered herein can be anyallotropic form of sulfur. The elemental sulfur considered hereintypically consists predominantly of crown-shaped S₈ molecules. However,numerous other forms and allotropes of sulfur are known, all of whichare considered herein. For example, by appropriate processingconditions, elemental sulfur containing S₆, S₇, S₉, S₁₀, S₁₁, S₁₂, or upto S₁₈ rings, or linear or branched forms, can be formed. In addition,the sulfur can be crystalline (e.g., of a rhombic or monoclinic spacegroup) or amorphous. The elemental sulfur is dissolved in a solvent toform the solution of elemental sulfur. The solvent is any solventcapable of dissolving elemental sulfur to the extent that a solution of,preferably, at least 1 wt % (and more preferably, 2, 5, 10, 15, or 20 wt%) sulfur is obtained. Some examples of such solvents include benzene,toluene, and carbon disulfide.

The driving force that determines the order in which pores are filled isthe adsorption energy, which increases with decreasing pore size. Due tothe higher adsorption energy of the micropores as compared to themesopores, the impregnation step generally first impregnates themicropores with sulfur before the mesopores become impregnated withsulfur. Once the micropores are filled, the small mesopores (i.e., ofabout 3 or 4 microns) will start to fill. Application of a heating(i.e., annealing) step after the impregnation step can further ensurethat the micropores are filled first.

After the bimodal porous carbon component (i.e., “carbon” or “carbonmaterial”) has been impregnated with sulfur, the solvent issubstantially removed from the sulfur-impregnated carbon (i.e., thesulfur-impregnated carbon is dried). By being “substantially removed” ismeant that at least 80%, and more preferably, at least 90%, 95%, or 98%of the solvent is removed. Any method of drying can be used, including,for example, air-drying at ambient temperature (e.g., 15-30° C.),application of a vacuum, and/or heating, e.g., for a suitable period oftime at a temperature of at least 30° C., but no more than 40° C., 50°C., 60° C., 70° C., 80° C., 90° C., or 100° C. After the drying step iscomplete, if desired, another impregnation step can be applied to thedried sulfur-impregnated carbon, followed by another drying step. Anynumber of impregnation-drying cycles can be applied to the carbonmaterial depending on the loading of sulfur desired; i.e., as the numberof impregnation-drying cycles applied to the carbon material isincreased, the sulfur loading increases. By knowing the concentration ofthe sulfur solution and the amount (i.e., mass or volume, asappropriate) of the solution used in each impregnation step, the amountof sulfur impregnated in the carbon material can be calculated bymultiplying the concentration of the solution and the amount of thesolution used. By weighing the carbon material before impregnation withsulfur, the amount of sulfur needed to achieve a particular sulfurloading can also be known.

The impregnation and drying process can be followed by an annealingprocess (i.e., a post-annealing step). Alternatively, the drying stepdescribed above can be omitted, and the impregnation step followeddirectly by an annealing process. The drying process may also bereplaced by an annealing process such that one or moreimpregnation-annealing cycles are applied to the porous carbon material.An annealing process is useful to remove residual amounts of solvents inthe sulfur-impregnated carbon material. The annealing process can alsobe beneficial for filling the pores because sulfur preferably melts atthe annealing temperature. The annealing process is preferably conductedat a temperature above 100° C., and more preferably at least at themelting point of sulfur (e.g., at least 115° C.), and below the boilingpoint of the elemental sulfur used, and more preferably, no more than400° C. For example, in different embodiments, an annealing temperatureof about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C.,180° C., 190° C., 200° C., 250° C., 300° C., 350° C., or 400° C. isused. Alternatively, the annealing temperature can be within a rangebounded by any two of these values. Preferably, the annealing process isconducted under an inert atmosphere environment. Some examples ofsuitable inert gases include nitrogen and argon.

In particular embodiments, the electron-conducting porous composition isin a suitable form and shape to be incorporated into or function as acathode for a lithium-sulfur battery. For purposes of functioning as acathode material in a lithium battery, the electron-conducting porouscomposition can be shaped as a film, coating, or layer. The film,coating, or layer can have any desirable thickness, such as a thicknessof 1, 2, 5, 10, 15, 20, 25, 50, 100, 200, 250, 500, 750, or 1 mm, or athickness within a particular range bounded by any two of the foregoingvalues.

The electron-conducting porous composition described above can be shapedby any of the methods known in the art, such as by pressing under aspecified load to form a desired shape, such as a disc. In conventionalpractice, the electron-conducting porous composition is combined with abinder to form a cathode composite (i.e., “composite”) with bettershaping qualities. The binder can be any of the binders known in theart, such as the fluoropolymer binders, of which polyvinylidenedifluoride (PVDF) is a prime example. The binder can be incorporatedinto the composite in any suitable amount, such as an amount of about,at least, above, up to, or less than, for example, 1, 2, 5, 10, 12, 15,20, 25, or 30 wt % by weight of the composite.

The electron-conducting porous composition can be included in thecathode composite in any suitable amount. In different embodiments, theelectron-conducting porous composition can be included in the compositein an amount of about, at least, above, up to, or less than, forexample, 5, 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75,80, 85, 90, 95, 98, or 100 wt % by weight of the composite, or an amountwithin a range bounded by any two of the foregoing values.

The composite may also include a desired amount of a conductive carbonmaterial. The conductive carbon can be any such materials known in theart, such as a conductive carbon black, graphite, glassy carbon, or afullerene, such as a carbon nanotube or spherical fullerene, such asbuckminsterfullerene (C₆₀). In embodiments where carbon nanotubes areincluded, the carbon nanotubes can be any of the carbon nanotubes knownin the art, including single-walled carbon nanotubes (SWNTs) ormulti-walled carbon nanotubes (MWNTs). In different embodiments, theconductive carbon can be included in the composite in an amount ofabout, at least, above, up to, or less than, for example, 1, 2, 5, 10,15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 wt % by weightof the composite, or an amount within a range bounded by any two of theforegoing values.

In another aspect, the invention is directed to a layer structure thatincludes a current collector material having coated thereon a layer ofthe nitrogen-sulfur-carbon composite material described above. The layerof composite material can have any suitable thickness, including any ofthe exemplary thicknesses described above. The current collectormaterial can be any conductive material with physical characteristicssuitable for use in lithium-sulfur batteries. Some examples of suitablecurrent collector materials include aluminum, nickel, cobalt, copper,zinc, conductive carbon forms, and alloys thereof. The current collectorcan be of any suitable shape and have any suitable surface morphology,including microstructural or nanostructural characteristics.

In another aspect, the invention is directed to a lithium-sulfur battery(i.e., “lithium ion battery” or “battery”) that contains theabove-described nitrogen-sulfur-carbon composite material as a cathodecomponent. The lithium-sulfur battery of the invention preferablypossesses the characteristic of being able to operate for at least 50,60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200,210, 220, 230, 240, or 250 cycles while maintaining a specific dischargecapacity (i.e., “discharge capacity” or “capacity”) of at least 350,400, 450, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, or 1500mA-hr/g.

As known in the art, the lithium-sulfur battery necessarily includes atleast a lithium anode (typically, a foil of lithium or lithium alloy), acathode (which in this case includes the nitrogen-sulfur-dopedelectron-conducting porous composition described above), separator, anda battery shell. The battery also typically includes current collectingplates (e.g., Cu or Al foil) for both anode and cathode. Duringoperation, the battery includes a lithium-containing electrolyte mediumin contact with the anode and cathode. The lithium-sulfur battery maycontain any of the components typically found in a lithium-sulfurbattery, such as described in, for example, U.S. Pat. Nos. 7,517,612 and8,252,461, the contents of which are incorporated herein by reference intheir entirety. To improve conductivity at the cathode, conductivecarbon material (e.g., carbon black, carbon fiber, or graphite) istypically included in the cathode material. The cathode material istypically admixed with an adhesive (e.g., PVDF, PTFE, and co-polymersthereof) in order to be properly molded as an electrode. In someembodiments, the cathode material may or may not also include a metaloxide, such as ZnO, CuO, SnO, or Mn₂O₃.

In one embodiment, the lithium-containing electrolyte medium is aliquid. In another embodiment, the lithium-containing electrolyte mediumis a solid. In yet another embodiment, the lithium-containingelectrolyte medium is a gel.

Preferably, the electrolyte medium includes a matrix material withinwhich is incorporated one or more lithium ion electrolytes. The lithiumion electrolyte can be any lithium ion electrolyte, and particularly,any of the lithium ion electrolytes known in the art.

In one embodiment, the lithium ion electrolyte (lithium salt) isnon-carbon-containing (i.e., inorganic). For example, the lithium ionelectrolyte can be a lithium ion salt of such counteranions as thehalides (e.g., chloride, bromide, or iodide), hexachlorophosphate (PCl₆⁻), hexafluorophosphate (PF₆ ⁻), perchlorate, chlorate, chlorite,perbromate, bromate, bromite, periodiate, iodate, aluminum fluorides(e.g., AlF₄ ⁻), aluminum chlorides (e.g., Al₂Cl₇ ⁻ and AlCl₄ ⁻),aluminum bromides (e.g., AlBr₄ ⁻), nitrate, nitrite, sulfate, sulfite,phosphate, phosphite, arsenate, hexafluoroarsenate (AsF₆ ⁻), antimonate,hexafluoroantimonate (SbF₆ ⁻), selenate, tellurate, tungstate,molybdate, chromate, silicate, the borates (e.g., borate, diborate,triborate, tetraborate), tetrafluoroborate, anionic borane clusters(e.g., B₁₀H₁₀ ²⁻ and B₁₂H₁₂ ²⁻), perrhenate, permanganate, ruthenate,perruthenate, and the polyoxometallates.

In another embodiment, the lithium ion electrolyte is carbon-containing(i.e., organic). The organic counteranion may, in one embodiment, lackfluorine atoms. For example, the lithium ion electrolyte can be alithium ion salt of such counteranions as carbonate, the carboxylates(e.g., formate, acetate, propionate, butyrate, valerate, lactate,pyruvate, oxalate, malonate, glutarate, adipate, decanoate, and thelike), the sulfonates (e.g., CH₃SO₃ ⁻, CH₃CH₂SO₃ ⁻, CH₃(CH₂)₂SO₃ ⁻,benzenesulfonate, toluenesulfonate, dodecylbenzenesulfonate, and thelike), the alkoxides (e.g., methoxide, ethoxide, isopropoxide, andphenoxide), the amides (e.g., dimethylamide and diisopropylamide),diketonates (e.g., acetylacetonate), the organoborates (e.g., BR₁R₂R₃R₄⁻, wherein R₁, R₂, R₃, R₄ are typically hydrocarbon groups containing 1to 6 carbon atoms), anionic carborane clusters, the alkylsulfates (e.g.,diethylsulfate), alkylphosphates (e.g., ethylphosphate ordiethylphosphate), dicyanamide (i.e., N(CN)₂ ⁻), and the phosphinates(e.g., bis-(2,4,4-trimethylpentyl)phosphinate). The organic counteranionmay, in another embodiment, include fluorine atoms. For example, thelithium ion electrolyte can be a lithium ion salt of such counteranionsas the fluorosulfonates (e.g., CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, CF₃(CF₂)₂SO₃ ⁻,CHF₂CF₂SO₃ ⁻, and the like), the fluoroalkoxides (e.g., CF₃O⁻, CF₃CH₂O⁻,CF₃CF₂O⁻, and pentafluorophenolate), the fluorocarboxylates (e.g.,trifluoroacetate and pentafluoropropionate), and the fluorosulfonimides(e.g., (CF₃SO₂)₂N⁻).

In some embodiments, any one or more classes or specific types oflithium salts are excluded from the electrolyte. In other embodiments, acombination of two or more lithium salts are included in theelectrolyte.

The lithium ion electrolyte is incorporated in the electrolyte mediumpreferably in an amount which imparts a suitable concentration oflithium ions and suitable level of conductivity to the electrolytemedium. The conductivity of the electrolyte medium can be, for example,at least 0.01 mS/cm (0.001 S/m) at an operating temperature of interest,and particularly at a temperature within 20-60° C. In differentembodiments, the lithium ion electrolyte is present in the electrolytein a concentration of precisely, about, at least, above, up to, or lessthan, for example, 0.1, 0.5, 1.0, 1.2, 1.5, 1.8, 2, 2.5, or 3 M, where“M” indicates a molarity concentration.

In some embodiments, the electrolyte medium further includes one or morehalide-containing additives (i.e., “halide additives”). The halideadditive can be any halide-containing ionic compound or material (i.e.,a salt). The halide considered herein can be, for example, fluoride(F⁻), chloride (Cl⁻), bromide (Br⁻), or iodide (I⁻), or a combinationthereof. The countercation can be any inorganic or organiccountercation. The inorganic countercation is typically either an alkali(i.e., Group I) or alkaline earth (i.e., Group II) metal cation.However, boron-group (i.e., Group III), carbon-group (i.e., Group IV,except those halocarbons which contain only a covalent instead of anionic carbon-halogen bond), nitrogen-group (i.e., Group V, except fornitrogen halides), and transition-metal halide compounds are alsoconsidered herein, as long as the halide compound or material is notcorrosive to the lithium anode. The halide additive is preferablycompletely soluble in the matrix material. The halide additive can be,for example, one or more lithium halides (e.g., LiF, LiCl, LiBr, LiI),sodium halides (e.g., NaF, NaCl, NaBr, NaI), potassium halides (e.g.,KF, KCl, KBr, KI), rubidium halides (e.g., RbF, RbCl, RbBr, RbI),magnesium halides (e.g., MgF₂, MgCl₂, MgBr₂, MgI₂), calcium halides(e.g., CaF₂, CaCl₂, CaBr₂, CaI₂), strontium halides (e.g., SrF₂, SrCl₂,SrBr₂, SrI₂), barium halides (e.g., BaF₂, BaCl₂, BaBr₂, BaI₂), Group IIIhalides (e.g., BF₃, BCl₃, AlF₃, AlCl₃, TIF, TlCl, and related compoundsor complexes), Group IV halides (e.g., SiCl₄, SnCl₂, SnCl₄), Group Vhalides (e.g., PCl₃, AsCl₃, SbCl₃, SbCl₅), transition-metal halides(e.g., TiCl₄, ZnCl₂), rare-earth halides (e.g., LaF₃, LaCl₃, CeF₃,CeCl₃), ammonium halides (e.g., NH₄F, NH₄Cl, NH₄Br, NH₄I), alkylammoniumhalides (e.g., MeNH₃Cl, Me₂NH₂Cl, Me₃NHCl, Me₄NCl, Et₄NCl, Bu₄NF,Bu₄NBr, where Me is methyl, Et is ethyl, and Bu is n-butyl), or acombination of any of these. In other embodiments, one or more of theforegoing groups of halide compounds or materials are excluded from theelectrolyte medium.

Preferably, the halide-containing additive is present in the electrolytemedium in at least a trace amount (e.g., at least 0.001 M or 0.001 m,where “M” indicates a molarity concentration and “m” indicates amolality concentration). In different embodiments, the halide additiveis present in a minimum amount of, for example, 0.01 M, 0.05 M, 0.1 M,0.2 M, 0.3 M, 0.4 M, 0.5 M, 0.6 M, 0.7 M, 0.8 M, 0.9 M, 1.0 M, 1.1 M,1.2 M, 1.3 M, 1.4 M, or 1.5 M. In other embodiments, the halide additiveis present in a maximum amount of, for example, 0.5 M, 0.6 M, 0.7 M, 0.8M, 0.9 M, 1.0 M, 1.1 M, 1.2 M, 1.3 M, 1.4 M, 1.5 M, 1.6 M, 1.7 M, 1.8 M,2.0 M, 2.1 M, 2.2 M, 2.3 M, 2.4 M, or 2.5 M. In other embodiments, thehalide additive is present in an amount within a range bounded by anycombination of minimum and maximum values given above, provided that theminimum value is less than the maximum value. Any of the concentrationsgiven above in terms of molarity (M) can alternatively be understood tobe molality (m) concentrations.

In the case of a liquid electrolyte medium, the matrix is a liquid,i.e., composed of one or more solvents. The solvent can be, for example,ionic (e.g., an ionic liquid) or non-ionic. The one or more solvents arepreferably non-reactive with the materials of the anode and the cathode,and furthermore, do not have a deleterious effect on the performancecharacteristics of the lithium ion battery.

In some embodiments, the one or more solvents are non-ionic solvents.The non-ionic solvent typically has a melting point no more or less than100, 90, 80, 70, 60, or 50° C., and more typically, below roomtemperature, i.e., below about 25° C., and more typically, up to or lessthan 20, 15, 10, 5, or 0° C. The non-ionic solvent, which is typicallyalso an aprotic polar solvent, can be, for example, a carbonate,sulfone, siloxane, silane, ether, ester, nitrile, sulfoxide, or amidesolvent, or a mixture thereof. Some examples of carbonate solventsinclude propylene carbonate (PC), ethylene carbonate (EC), butylenecarbonate (BC), chloroethylene carbonate, fluorocarbonate solvents(e.g., fluoroethylene carbonate and trifluoromethyl propylenecarbonate), as well as the dialkylcarbonate solvents, such as dimethylcarbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC),ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), and ethylpropyl carbonate (EPC). Some examples of sulfone solvents include methylsulfone, ethyl methyl sulfone, methyl phenyl sulfone, methyl isopropylsulfone (MiPS), propyl sulfone, butyl sulfone, tetramethylene sulfone(sulfolane), phenyl vinyl sulfone, allyl methyl sulfone, methyl vinylsulfone, divinyl sulfone (vinyl sulfone), diphenyl sulfone (phenylsulfone), dibenzyl sulfone (benzyl sulfone), vinylene sulfone, butadienesulfone, 4-methoxyphenyl methyl sulfone, 4-chlorophenyl methyl sulfone,2-chlorophenyl methyl sulfone, 3,4-dichlorophenyl methyl sulfone,4-(methylsulfonyl)toluene, 2-(methylsulfonyl)ethanol, 4-bromophenylmethyl sulfone, 2-bromophenyl methyl sulfone, 4-fluorophenyl methylsulfone, 2-fluorophenyl methyl sulfone, 4-aminophenyl methyl sulfone, asultone (e.g., 1,3-propanesultone), and sulfone solvents containingether groups (e.g., 2-methoxyethyl(methyl)sulfone and2-methoxyethoxyethyl(ethyl)sulfone). Some examples of siloxane solventsinclude hexamethyldisiloxane (HMDS), 1,3-divinyltetramethyldisiloxane,the polysiloxanes, and polysiloxane-polyoxyalkylene derivatives. Someexamples of silane solvents include methoxytrimethylsilane,ethoxytrimethylsilane, dimethoxydimethylsilane, methyltrimethoxysilane,and 2-(ethoxy)ethoxytrimethylsilane. Some examples of ether solventsinclude diethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane,1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran,tetrahydropyran, diglyme, triglyme, 1,3-dioxolane, and the fluorinatedethers (e.g., mono-, di-, tri-, tetra-, penta-, hexa- and per-fluoroderivatives of any of the foregoing ethers). Some examples of estersolvents include 1,4-butyrolactone, ethylacetate, methylpropionate,ethylpropionate, propylpropionate, methylbutyrate, ethylbutyrate, theformates (e.g., methyl formate, ethyl formate, or propyl formate), andthe fluorinated esters (e.g., mono-, di-, tri-, tetra-, penta-, hexa-and per-fluoro derivatives of any of the foregoing esters). Someexamples of nitrile solvents include acetonitrile, propionitrile, andbutyronitrile. Some examples of sulfoxide solvents include dimethylsulfoxide, ethyl methyl sulfoxide, diethyl sulfoxide, methyl propylsulfoxide, and ethyl propyl sulfoxide. Some examples of amide solventsinclude formamide, N,N-dimethylformamide, N,N-diethylformamide,acetamide, dimethylacetamide, diethylacetamide, gamma-butyrolactam, andN-methylpyrrolidone. The non-ionic solvent can be included in anon-additive or additive amount, such as any of the exemplary amountsprovided above for the ionic liquids. The non-ionic solvent may also be,for example, an organochloride (e.g., methylene chloride, chloroform,1,1-trichloroethane), ketone (e.g., acetone, 2-butanone), organoethers(e.g., diethyl ether, tetrahydrofuran, and dioxane),hexamethylphosphoramide (HMPA), N-methylpyrrolidinone (NMP),1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), and propyleneglycol monomethyl ether acetate (PGMEA).

In some embodiments, the one or more solvents contain one or moreoxyether (i.e., carbon-oxygen-carbon) groups. The one or more solventscan be ether solvents, i.e., polar aprotic solvents formulated ashydrocarbons except that they contain one or more carbon-oxygen-carbongroups (e.g., one, two, three, four, five, or six C—O—C groups) in theabsence of any other chemical groups. The ether solvents typicallycontain at least three, four, five, six, seven, or eight carbon atoms,and up to nine, ten, eleven, twelve, or higher number of carbon atoms,and can be acyclic or cyclic. The ether solvent may also be saturated,or alternatively, unsaturated (i.e., by the presence of one or morecarbon-carbon double or triple bonds).

Some examples of acyclic ether solvents containing one oxygen atominclude diethyl ether, di(n-propyl)ether, diisopropyl ether, diisobutylether, methyl(t-butyl)ether, and anisole. Some examples of acyclic ethersolvents containing two or more oxygen atoms include ethylene glycoldimethyl ether (i.e., dimethoxyethane, or DME, or glyme), diethyleneglycol dimethyl ether (diglyme), triethylene glycol dimethyl ether(triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Theforegoing exemplary acyclic ether solvents all contain methyl groups asendcapping groups. However, any hydrocarbon endcapping groups aresuitable. Some common endcapping groups aside from methyl groupsinclude, allyl, vinyl, n-propyl, isopropyl, n-butyl, isobutyl,sec-butyl, and t-butyl groups.

Some examples of cyclic ether solvents containing one oxygen atominclude propylene oxide, 2,3-epoxybutane (i.e., 2,3-dimethyloxirane),oxetane, tetrahydrofuran (THF), furan, tetrahydropyran, and pyran. Someexamples of cyclic ether solvents containing two or more oxygen atomsinclude 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, and thecrown ethers.

In some embodiments, the electrolyte medium includes a non-polar liquid.Some examples of non-polar liquids include the liquid hydrocarbons, suchas a pentane, hexane, heptane, octane, pentene, hexene, heptene, octene,benzene, toluene, or xylene. In another embodiment, non-polar liquidsare excluded from the electrolyte medium.

In some embodiments, the one or more solvents include at least onesolvent which is an aprotic ether solvent that has a tendency topolymerize, particularly in the presence of a halide (as provided, forexample, when a halide-containing additive is included). Particularlypreferred in this respect are the cyclic ethers, and in particular, oneor a combination of solvents selected from 1,3-dioxolane,dimethoxyethane, and 1,3,5-trioxane. The polymerization of thesesolvents during cycling in the presence of a halide-containing additivemay improve the cycling performance of lithium-sulfur batteries.

In other embodiments, the electrolyte includes one or more ionicliquids. The ionic liquid can be, for example, of the formula Y⁺X⁻,wherein Y⁺ is a cationic component of the ionic liquid and X⁻ is ananionic component of the ionic liquid. The formula (Y⁺)(X⁻) is meant toencompass a cationic component (Y⁺) having any valency of positivecharge, and an anionic component (X⁻) having any valency of negativecharge, provided that the charge contributions from the cationic portionand anionic portion are counterbalanced in order for charge neutralityto be preserved in the ionic liquid molecule. More specifically, theformula (Y⁺)(X⁻) is meant to encompass the more generic formula(Y^(+a))_(y)(X^(−b))_(x), wherein the variables a and b are,independently, non-zero integers, and the subscript variables x and yare, independently, non-zero integers, such that a·y=b·x (wherein theperiod placed between variables indicates multiplication of thevariables). The foregoing generic formula encompasses numerous possiblesub-formulas, such as, for example, (Y⁺)(X⁻), (Y⁺²)(X⁻)₂, (Y⁺)₂(X⁻²),(Y⁺²)₂(X⁻²)₂, (Y⁺³)(X⁻)₃, (Y⁺)₃(X⁻³), (Y⁺³)₂(X⁻²)₃, and (Y⁺²)₃(X⁻³)₂.For simplicity, numerous embodiments of ionic liquids, described below,designate the anion as X⁻, which in its strict sense indicates amonovalent anion. However, the anion designated as X⁻ is meant toencompass an anion of any valency, such as any of the valenciesdescribed above and further below, unless otherwise specified.

The ionic liquid is typically a liquid at room temperature (e.g., 15,18, 20, 22, 25, or 30° C.) or lower. However, in some embodiments, theionic liquid may become a liquid at a higher temperature than 30° C. ifit is used at an elevated temperature that melts the ionic liquid. Thus,in some embodiments, the ionic liquid may have a melting point of up toor less than 100, 90, 80, 70, 60, 50, 40, or 30° C. In otherembodiments, the ionic liquid is a liquid at or below 10, 5, 0, −10,−20, −30, or −40° C.

The density of the ionic liquid is generally above 1.2 g/mL at anoperating temperature of interest, and particularly at a temperaturewithin 20-30° C. In different embodiments, the density of the ionicliquid can be at least 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, or 1.8 g/mL, or aparticular range bounded by any two of these values.

The viscosity of the ionic liquid is typically no more than 50,000centipoise (50,000 cP) at an operating temperature of interest, andparticularly at a temperature within 20-30° C. More typically, theviscosity of the ionic liquid is no more than about 25,000 cP, 10,000cP, 5,000 cP, 2,000 cP, 1,000 cP, 800 cP, 700 cP, 600 cP, 500 cP, 400cP, 300 cP, 200 cP, 100 cP, or 50 cP.

In one set of embodiments, the cationic portion (Y⁺) of the ionic liquidY⁺X⁻ is an ammonium species. In some embodiments, the ammonium cationportion includes a heterocyclic ring having a positively-charged ringnitrogen atom. The heterocyclic ring having a positively-charged ringnitrogen atom can be monocyclic, bicyclic, tricyclic, or a higher cyclic(polycyclic) ring system. Some examples of a heterocyclic ring having apositively-charged ring nitrogen atom include imidazolium, pyridinium,pyrazinium, pyrrolidinium, piperidinium, piperazinium, morpholinium,pyrrolium, pyrazolium, pyrimidinium, triazolium, oxazolium, thiazolium,triazinium, and cyclic guanidinium rings. Any of the foregoing cationicrings may be bound or fused with one or more other saturated orunsaturated (e.g., aromatic) rings, such as a benzene, cyclohexane,cyclohexene, pyridine, pyrazine, pyrrolidine, piperidine, piperazine,pyrrole, pyrazole, pyrimidine, or indole rings. Some examples of fusedcharged rings include benzimidazolium, pyrrolo[1,2-a]pyrimidinium,indolium, quinolinium, quinazolinium, quinoxalinium,5,6,7,8-tetrahydroimidazo[1,2-a]pyridine, and H-imidazo[1,2-a]pyridine.Any of the foregoing cationic rings may be substituted by one or morehydrocarbon groups (R) as further described below. Typically, at leastone ring nitrogen atom is substituted with a hydrocarbon group (R) toprovide the positive charge. Ionic liquids containing any of theforegoing cationic components are either commercially available or canbe synthesized by procedures well-known in the art, as evidenced by, forexample, T. L. Greaves, et al., “Protic Ionic Liquids: Properties andApplications”, Chem. Rev., 108, pp. 206-237 (2008), the contents ofwhich are herein incorporated by reference in their entirety. Any of theionic liquids described in the foregoing reference may be used herein.

Some general examples of imidazolium-based ionic liquids according toFormula (1) include 1,3-dimethylimidazolium⁺X⁻,1,2,3-trimethylimidazolium⁺X⁻, 2-ethyl-1,3-dimethylimidazolium⁺X⁻,2-n-propyl-1,3-dimethylimidazolium⁺X⁻,2-n-butyl-1,3-dimethylimidazolium⁺X⁻,1-ethyl-2,3-dimethylimidazolium⁺X⁻,1-n-propyl-2,3-dimethylimidazolium⁺X⁻,1-n-butyl-2,3-dimethylimidazolium⁺X⁻, 1-methyl-3-ethylimidazolium⁺X⁻,1-methyl-3-n-propylimidazolium⁺X⁻, 1-methyl-3-isopropylimidazolium⁺X⁻,1-methyl-3-n-butylimidazolium⁺X⁻ (i.e., BMIM⁺X⁻),1-methyl-3-isobutylimidazolium⁺X⁻, 1-methyl-3-sec-butylimidazolium⁺X⁻,1-methyl-3-t-butylimidazolium⁺X⁻, 1,3-diethylimidazolium⁺X⁻,1-ethyl-3-n-propylimidazolium⁺X⁻, 1-ethyl-3-isopropylimidazolium⁺X⁻,1-ethyl-3-n-butylimidazolium⁺X⁻, 1-ethyl-3-isobutylimidazolium⁺X⁻,1-ethyl-3-sec-butylimidazolium⁺X⁻, 1-ethyl-3-t-butylimidazolium⁺X⁻,1,3-di-n-propylimidazolium⁺X⁻, 1-n-propyl-3-isopropylimidazolium⁺X⁻,1-n-propyl-3-n-butylimidazolium⁺X⁻, 1-n-propyl-3-isobutylimidazolium⁺X⁻,1-n-propyl-3-sec-butylimidazolium⁺X⁻,1-n-propyl-3-t-butylimidazolium⁺X⁻, 1,3-diisopropylimidazolium⁺X⁻,1-isopropyl-3-n-butylimidazolium⁺X⁻,1-isopropyl-3-isobutylimidazolium⁺X⁻,1-isopropyl-3-sec-butylimidazolium⁺X⁻,1-isopropyl-3-t-butylimidazolium⁺X⁻, 1,3-di-n-butylimidazolium⁺X⁻,1-n-butyl-3-isobutylimidazolium⁺X⁻, 1-n-butyl-3-sec-butylimidazolium⁺X⁻,1-n-butyl-3-t-butylimidazolium⁺X⁻, 1,3-diisobutylimidazolium⁺X⁻,1-isobutyl-3-sec-butylimidazolium⁺X⁻,1-isobutyl-3-t-butylimidazolium⁺X⁻, 1,3-di-sec-butylimidazolium⁺X⁻,1-sec-butyl-3-t-butylimidazolium⁺X⁻, 1,3-di-t-butylimidazolium⁺X⁻,1-methyl-3-pentylimidazolium⁺X⁻, 1-methyl-3-hexylimidazolium⁺X⁻,1-methyl-3-heptylimidazolium⁺X⁻, 1-methyl-3-octylimidazolium⁺X⁻,1-methyl-3-decylimidazolium⁺X⁻, 1-methyl-3-dodecylimidazolium⁺X⁻,1-methyl-3-tetradecylimidazolium⁺X⁻, 1-methyl-3-hexadecylimidazolium⁺X⁻,1-methyl-3-octadecylimidazolium⁺X⁻,1-(2-hydroxyethyl)-3-methylimidazolium⁺X⁻, and1-allyl-3-methylimidazolium⁺X⁻.

Some examples of pyridinium ionic liquids includeN-methyl-4-methylpyridinium⁺X⁻, N-ethyl-4-methylpyridinium⁺X⁻,N-methyl-4-ethylpyridinium⁺X⁻, N-methyl-4-isopropylpyridinium⁺X⁻,N-isopropyl-4-methylpyridinium⁺X⁻, and N-octyl-4-methylpyridinium⁺X⁻.

Some examples of quaternary ammonium ionic liquids includemethylammonium⁺X⁻, dimethylammonium⁺X⁻, trimethylammonium⁺X⁻,tetramethylammonium⁺X⁻, ethylammonium⁺X⁻, ethyltrimethylammonium⁺X⁻,diethylammonium⁺X⁻, triethylammonium⁺X⁻, tetraethylammonium⁺X⁻,n-propylammonium⁺X⁻, n-propyltrimethylammonium⁺X⁻, isopropylammonium⁺X⁻,n-butylammonium⁺X⁻, n-butyltrimethylammonium⁺X⁻,n-butylmethylammonium⁺X⁻, di-(n-butyl)dimethylammonium⁺X⁻,tri-(n-butyl)methylammonium⁺X⁻, n-pentylammonium⁺X⁻,n-pentyltrimethylammonium⁺X⁻, tri-(n-pentyl)methylammonium⁺X⁻,n-hexylammonium⁺X⁻, n-hexyltrimethylammonium⁺X⁻,tri-(n-hexyl)methylammonium⁺X⁻, n-heptylammonium⁺X⁻,n-heptyltrimethylammonium⁺X⁻, tri-(n-heptyl)methylammonium⁺X⁻,n-octylammonium⁺X⁻, n-octyltrimethylammonium⁺X⁻,tri-(n-octyl)methylammonium⁺X⁻, benzyltrimethylammonium⁺X⁻, choline⁺X⁻,2-hydroxyethylammonium⁺X⁻, allylammonium⁺X⁻, allyltrimethylammonium⁺X⁻,[(2-methacryloxy)ethyl]-trimethylammonium⁺X⁻, and(4-vinylbenzyl)trimethylammonium⁺X⁻.

Some examples of piperidinium-based ionic liquids include1,1-dimethylpiperidinium⁺X⁻, 1-methyl-1-ethylpiperidinium⁺X⁻,1-methyl-1-propylpiperidinium⁺X⁻, 1-methyl-1-butylpiperidinium⁺X⁻,1-methyl-1-isobutylpiperidinium⁺X⁻, 1-methyl-1-pentylpiperidinium⁺X⁻,1-methyl-1-hexylpiperidinium⁺X⁻, 1-methyl-1-heptylpiperidinium⁺X⁻,1-methyl-1-octylpiperidinium⁺X⁻, 1-methyl-1-decylpiperidinium⁺X⁻,1-methyl-1-dodecylpiperidinium⁺X⁻, 1-methyl-1-tetradecylpiperidinium⁺X⁻,1-methyl-1-hexadecylpiperidinium⁺X⁻,1-methyl-1-octadecylpiperidinium⁺X⁻, 1,1-diethylpiperidinium⁺X⁻,1,1-dipropylpiperidinium⁺X⁻, 1,1-dibutylpiperidinium⁺X⁻, and1,1-diisobutylpiperidinium⁺X⁻.

Some examples of pyrrolidinium-based ionic liquids include1,1-dimethylpyrrolidinium⁺X⁻, 1-methyl-1-ethylpyrrolidinium⁺X⁻,1-methyl-1-propylpyrrolidinium⁺X⁻, 1-methyl-1-butylpyrrolidinium⁺X⁻,1-methyl-1-isobutylpyrrolidinium⁺X⁻, 1-methyl-1-pentylpyrrolidinium⁺X⁻,1-methyl-1-hexylpyrrolidinium⁺X⁻, 1-methyl-1-heptylpyrrolidinium⁺X⁻,1-methyl-1-octylpyrrolidinium⁺X⁻, 1-methyl-1-decylpyrrolidinium⁺X⁻,1-methyl-1-dodecylpyrrolidinium⁺X⁻,1-methyl-1-tetradecylpyrrolidinium⁺X⁻,1-methyl-1-hexadecylpyrrolidinium⁺X⁻,1-methyl-1-octadecylpyrrolidinium⁺X⁻, 1,1-diethylpyrrolidinium⁺X⁻,1,1-dipropylpyrrolidinium⁺X⁻, 1,1-dibutylpyrrolidinium⁺X⁻, and1,1-diisobutylpyrrolidinium⁺X⁻.

In other embodiments, the ionic liquid is a cyclic guanidinium-basedionic liquid. The cyclic guanidinium-based ionic liquid can have any ofthe structures known in the art, including those described in U.S. Pat.No. 8,129,543 and M. G. Bogdanov, et al., Z. Naturforsch, 65b, pp.37-48, 2010, the contents of which are herein incorporated by referencein their entirety.

In other embodiments, the ionic liquid is a phosphonium-based ionicliquid. Some general examples of phosphonium-based ionic liquids includetetramethylphosphonium⁺X⁻, tetraethylphosphonium⁺X⁻,tetrapropylphosphonium⁺X⁻, tetrabutylphosphonium⁺X⁻,tetrapentylphosphonium⁺X⁻, tetrahexylphosphonium⁺X⁻,tetraheptylphosphonium⁺X⁻, tetraoctylphosphonium⁺X⁻,tetranonylphosphonium⁺X⁻, tetradecylphosphonium⁺X⁻,tetraphenylphosphonium⁺X⁻, tetrabenzylphosphonium⁺X⁻,ethyltrimethylphosphonium⁺X⁻, n-propyltrimethylphosphonium⁺X⁻,butyltrimethylphosphonium⁺X⁻, dibutyldimethylphosphonium⁺X⁻,tributylmethylphosphonium⁺X⁻, butyltriethylphosphonium⁺X⁻,dibutyldiethylphosphonium⁺X⁻, tributylethylphosphonium⁺X⁻,triisobutylmethylphosphonium⁺X⁻, tributylhexylphosphonium⁺X⁻,tributylheptylphosphonium⁺X⁻, tributyloctylphosphonium⁺X⁻,tributyldecylphosphonium⁺X⁻, tributyldodecylphosphonium⁺X⁻,tributyltetradecylphosphonium⁺X⁻, tributylhexadecylphosphonium⁺X⁻,hexyltrimethylphosphonium⁺X⁻, dihexyldimethylphosphonium⁺X⁻,trihexylmethylphosphonium⁺X⁻, hexyltriethylphosphonium⁺X⁻,trihexyloctylphosphonium⁺X⁻, trihexyldecylphosphonium⁺X⁻,trihexyldodecylphosphonium⁺X⁻, trihexyltetradecylphosphonium⁺X⁻,trihexylhexadecylphosphonium⁺X⁻, octyltrimethylphosphonium⁺X⁻,dioctyldimethylphosphonium⁺X⁻, trioctylmethylphosphonium⁺X⁻,octyltriethylphosphonium⁺X⁻, trioctyldecylphosphonium⁺X⁻,trioctyldodecylphosphonium⁺X⁻, trioctyltetradecylphosphonium⁺X⁻,trioctylhexadecylphosphonium⁺X⁻, tridecylmethylphosphonium⁺X⁻,phenyltrimethylphosphonium⁺X⁻, phenyltriethylphosphonium⁺X⁻,phenyltripropylphosphonium⁺X⁻, phenyltributylphosphonium⁺X⁻,diphenyldimethylphosphonium⁺X⁻, triphenylmethylphosphonium⁺X⁻, andbenzyltrimethylphosphonium⁺X⁻.

In other embodiments, the ionic liquid is a sulfonium-based ionicliquid. Some general examples of sulfonium-based ionic liquids includetrimethylsulfonium⁺X⁻, dimethylethylsulfonium⁺X⁻,diethylmethylsulfonium⁺X⁻, triethylsulfonium⁺X⁻,dimethylpropylsulfonium⁺X⁻, dipropylmethylsulfonium⁺X⁻,tripropylsulfonium⁺X⁻, dimethylbutylsulfonium⁺X⁻,dibutylmethylsulfonium⁺X⁻, tributylsulfonium⁺X⁻,dimethylhexylsulfonium⁺X⁻, dihexylmethylsulfonium⁺X⁻,trihexylsulfonium⁺X⁻, dimethyloctylsulfonium⁺X⁻,dioctylmethylsulfonium⁺X⁻, and trioctylsulfonium⁺X⁻.

The counteranion (X⁻) of the ionic liquid is any counteranion which,when associated with the cationic component, permits the resulting ioniccompound to behave as an ionic liquid. As known in the art, thecomposition and structure of the counteranion strongly affects theproperties (e.g., melting point, volatility, stability, viscosity,hydrophobicity, and so on) of the ionic liquid. In some embodiments, thecounteranion is structurally symmetrical, while in other embodiments,the counteranion is structurally asymmetrical.

In one embodiment, the counteranion (X⁻) of the ionic liquid isnon-carbon-containing (i.e., inorganic). The inorganic counteranion may,in one embodiment, lack fluorine atoms. Some examples of suchcounteranions include chloride, bromide, iodide, hexachlorophosphate(PCl₆ ⁻), perchlorate, chlorate, chlorite, cyanate, isocyanate,thiocyanate, isothiocyanate, perbromate, bromate, bromite, periodiate,iodate, dicyanamide (i.e., N(CN)₂ ⁻), tricyanamide (i.e., N(CN)₃ ⁻),aluminum chlorides (e.g., Al₂Cl₇ ⁻ and AlCl₄ ⁻), aluminum bromides(e.g., AlBr₄ ⁻), nitrate, nitrite, sulfate, sulfite, hydrogensulfate,hydrogensulfite, phosphate, hydrogenphosphate (HPO₄ ²⁻),dihydrogenphosphate (H₂PO₄ ⁻), phosphite, arsenate, antimonate,selenate, tellurate, tungstate, molybdate, chromate, silicate, theborates (e.g., borate, diborate, triborate, tetraborate), anionic boraneand carborane clusters (e.g., B₁₀H₁₀ ²⁻ and B₁₂H₁₂ ²⁻), perrhenate,permanganate, ruthenate, perruthenate, and the polyoxometallates. Theinorganic counteranion may, in another embodiment, include fluorineatoms. Some examples of such counteranions include fluoride, bifluoride(HF₂ ⁻), hexafluorophosphate (PF₆ ⁻), fluorophosphate (PO₃F₂ ⁻),tetrafluoroborate (BF₄ ⁻), aluminum fluorides (e.g., AlF₄ ⁻),hexafluoroarsenate (AsF₆ ⁻), and hexafluoroantimonate (SbF₆ ⁻).

In another embodiment, the counteranion (X⁻) of the ionic liquid iscarbon-containing (i.e., organic). The organic counteranion may, in oneembodiment, lack fluorine atoms. Some examples of such counteranionsinclude carbonate, bicarbonate, the carboxylates (e.g., formate,acetate, propionate, butyrate, valerate, lactate, pyruvate, oxalate,malonate, glutarate, adipate, decanoate, salicylate, ibuprofenate, andthe like), the sulfonates (e.g., CH₃SO₃ ⁻, CH₃CH₂SO₃ ⁻, CH₃(CH₂)₂SO₃ ⁻,benzenesulfonate, toluenesulfonate, dodecylbenzenesulfonate, docusate,and the like), the alkoxides (e.g., methoxide, ethoxide, isopropoxide,phenoxide, and glycolate), the amides (e.g., dimethylamide anddiisopropylamide), diketonates (e.g., acetylacetonate), theorganoborates (e.g., BR₁R₂R₃R₄ ⁻, wherein R₁, R₂, R₃, R₄ are typicallyhydrocarbon groups containing 1 to 6 carbon atoms), the alkylsulfates(e.g., diethylsulfate), alkylphosphates (e.g., ethylphosphate ordiethylphosphate), and the phosphinates (e.g.,bis-(2,4,4-trimethylpentyl)phosphinate). The organic counteranion may,in another embodiment, include fluorine atoms. Some examples of suchcounteranions include the fluorosulfonates (e.g., CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻,CF₃(CF₂)₂SO₃ ⁻, CHF₂CF₂SO₃ ⁻, and the like), the fluoroalkoxides (e.g.,CF₃O⁻, CF₃CH₂O⁻, CF₃CF₂O⁻, and pentafluorophenolate), thefluorocarboxylates (e.g., trifluoroacetate and pentafluoropropionate),and the fluorosulfonylimides (e.g., (CF₃SO₂)₂N⁻).

In particular embodiments, the counteranion (X⁻) of the ionic liquid hasa structure according to the following general formula:

In Formula (1) above, subscripts m and n are independently 0 or aninteger of 1 or above. Subscript p is 0 or 1, provided that when p is 0,the group —N—SO₂—(CF₂)_(n)CF₃ subtended by p is replaced with an oxideatom connected to the sulfur atom (S).

In one embodiment of Formula (1), subscript p is 1, so that Formula (9)reduces to the chemical formula:

In one embodiment of Formula (1a), m and n are the same number, therebyresulting in a symmetrical counteranion. In another embodiment ofFormula (1a), m and n are not the same number, thereby resulting in anasymmetrical counteranion.

In a first set of embodiments of Formula (1a), m and n are independentlyat least 0 and up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11. When m and nare both 0, the resulting anion has the formula F₃CSO₂NSO₂CF₃, i.e.,bis-(trifluoromethylsulfonyl)imide, or Tf₂N⁻. In another embodiment, mand n are not both 0. For example, in a particular embodiment, m is 0while n is a value of 1 or above (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,or 11). Some examples of such anions include F₃CSO₂NSO₂CF₂CF₃,F₃CSO₂NSO₂ (CF₂)₂CF₃, F₃CSO₂NSO₂ (CF₂)₃CF₃, F₃CSO₂NSO₂ (CF₂)₄CF₃,F₃CSO₂NSO₂ (CF₂)₅CF₃, and so on, wherein it is understood that, in theforegoing examples, the negative sign indicative of a negative charge(i.e., “−”) in the anion has been omitted for the sake of clarity.

In a second set of embodiments of Formula (1a), m and n areindependently at least 1 and up to 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11.For example, in a particular embodiment, m is 1 while n is a value of 1or above (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples ofsuch anions include N[SO₂CF₂CF₃]₂ (i.e., “BETI⁻”), F₃CF₂CSO₂NSO₂(CF₂)₂CF₃, F₃CF₂CSO₂NSO₂ (CF₂)₃CF₃, F₃CF₂CSO₂NSO₂ (CF₂)₄CF₃,F₃CF₂CSO₂NSO₂ (CF₂)₅CF₃, and so on.

In a third set of embodiments of Formula (1a), m and n are independentlyat least 2 and up to 3, 4, 5, 6, 7, 8, 9, 10, or 11. For example, in aparticular embodiment, m is 2 while n is a value of 2 or above (e.g., 2,3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of such anions includeN[SO₂ (CF₂)₂CF₃]₂, F₃C(F₂C)₂SO₂NSO₂ (CF₂)₃CF₃, F₃C(F₂C)₂SO₂NSO₂(CF₂)₄CF₃, F₃C(F₂C)₂SO₂NSO₂ (CF₂)₅CF₃, and so on.

In a fourth set of embodiments of Formula (1a), m and n areindependently at least 3 and up to 4, 5, 6, 7, 8, 9, 10, or 11. Forexample, in a particular embodiment, m is 3 while n is a value of 3 orabove (e.g., 3, 4, 5, 6, 7, 8, 9, 10, or 11). Some examples of suchanions include N[SO₂ (CF₂)₃CF₃]₂, F₃C(F₂C)₃SO₂NSO₂ (CF₂)₄CF₃,F₃C(F₂C)₃SO₂NSO₂ (CF₂)₅CF₃, F₃C(F₂C)₃SO₂NSO₂ (CF₂)₆CF₃, F₃C(F₂C)₃SO₂NSO₂(CF₂)₇CF₃, and so on.

In a fifth set of embodiments of Formula (1a), m and n are independentlyat least 4 and up to 5, 6, 7, 8, 9, 10, or 11. For example, in aparticular embodiment, m is 4 while n is a value of 4 or above (e.g., 4,5, 6, 7, 8, 9, 10, or 11). Some examples of such anions include N[SO₂(CF₂)₄CF₃]₂, F₃C(F₂C)₄SO₂NSO₂ (CF₂)₅CF₃, F₃C(F₂C)₄SO₂NSO₂ (CF₂)₆CF₃,F₃C(F₂C)₄SO₂NSO₂ (CF₂)₇CF₃, F₃C(F₂C)₄SO₂NSO₂ (CF₂)₈CF₃, and so on.

In a sixth set of embodiments of Formula (1a), m and n are independentlyat least 5 and up to 6, 7, 8, 9, 10, or 11. For example, in a particularembodiment, m is 5 while n is a value of 5 or above (e.g., 5, 6, 7, 8,9, 10, or 11). Some examples of such anions include N[SO₂ (CF₂)₅CF₃]₂,F₃C(F₂C)₅SO₂NSO₂ (CF₂)₆CF₃, F₃C(F₂C)₅SO₂NSO₂ (CF₂)₇CF₃, F₃C(F₂C)₅SO₂NSO₂(CF₂)₈CF₃, F₃C(F₂C)₅SO₂NSO₂ (CF₂)₉CF₃, and so on.

In a seventh set of embodiments of Formula (1a), m and n areindependently at least 6 and up to 7, 8, 9, 10, or 11. For example, in aparticular embodiment, m is 6 while n is a value of 6 or above (e.g., 6,7, 8, 9, 10, or 11). Some examples of such anions include N[SO₂(CF₂)₆CF₃]₂, F₃C(F₂C)₆SO₂NSO₂ (CF₂)₇CF₃, F₃C(F₂C)₆SO₂NSO₂ (CF₂)₈CF₃,F₃C(F₂C)₆SO₂NSO₂ (CF₂)₉CF₃, F₃C(F₂C)₆SO₂NSO₂ (CF₂)₁₀CF₃, and so on.

In other embodiments of Formula (1a), m abides by one or a number ofalternative conditions set forth in one of the foregoing sevenembodiments while n abides by one or a number of alternative conditionsset forth in another of the foregoing seven embodiments.

In another embodiment of Formula (1), subscript p is 0, so that Formula(1) reduces to the chemical formula:

In different exemplary embodiments of Formula (1b), m can be 0 or above(e.g., up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 1 or above (e.g., upto 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11), 2 or above (e.g., up to 3, 4, 5,6, 7, 8, 9, 10, or 11), 3 or above (e.g., up to 4, 5, 6, 7, 8, 9, 10, or11), 4 or above (e.g., up to 5, 6, 7, 8, 9, 10, or 11), 5 or above(e.g., up to 6, 7, 8, 9, 10, or 11), 6 or above (e.g., up to 7, 8, 9,10, or 11), 7 or above (e.g., up to 8, 9, 10, 11, or 12), 8 or above(e.g., up to 9, 10, 11, or 12), or 9 or above (e.g., up to 10, 11, 12,13, 14, 15, or 16). Some examples of such anions include F₃CSO₃ ⁻ (i.e.,“triflate” or “TfO⁻”), F₃CF₂CSO₃ ⁻, F₃C(F₂C)₂SO₃ ⁻, F₃C(F₂C)₃SO₃ ⁻(i.e., “nonaflate” or “NfO⁻”), F₃C(F₂C)₄SO₃ ⁻, F₃C(F₂C)₅SO₃ ⁻,F₃C(F₂C)₆SO₃ ⁻, F₃C(F₂C)₇SO₃ ⁻, F₃C(F₂C)₈SO₃ ⁻, F₃C(F₂C)₉SO₃ ⁻,F₃C(F₂C)₁₀SO₃ ⁻, F₃C(F₂C)₁₁SO₃ ⁻, and so on.

In some embodiments, any one or more classes or specific types of anionsdescribed above are excluded from the ionic liquid. In otherembodiments, a combination of anions is used in the ionic liquid.

The ionic liquid can be of any suitable purity level. Preferably, theionic liquid has a purity at least or greater than 90%, 95%, 96%, 97%,98%, 99%, 99.5%, or 99.9%. The ionic liquid is preferably substantiallydevoid of salt byproducts (e.g., LiNO₃) that are typically producedduring synthesis of the ionic liquid. In preferred embodiments, it isdesirable that the ionic liquid contains less than 1% by weight of saltbyproducts, and more preferably, less than 0.5%, 0.1%, 0.01%, or even0.001% by weight of salt byproducts.

The electrolyte medium generally excludes a protic liquid. Proticliquids are generally reactive with the lithium anode. Some examples ofpolar protic solvents that are preferably excluded include water, thealcohols (e.g., methanol, ethanol, isopropanol, n-butanol, t-butanol,the pentanols, hexanols, octanols, or the like), diols (e.g., ethyleneglycol, diethylene glycol, triethylene glycol), and protic amines (e.g.,ethylenediamine, ethanolamine, diethanolamine, and triethanolamine).

A non-ionic solvent additive may or may not also be included in theelectrolyte. The non-ionic solvent additive can be, for example, any ofthe non-ionic solvent additives described above, but is more typicallyany such solvent that possesses one or more unsaturated groupscontaining a carbon-carbon double bond and/or one or more halogen atoms.Some particular examples of solvent additives include vinylene carbonate(VC), vinyl ethylene carbonate, allyl ethyl carbonate, vinyl acetate,divinyl adipate, acrylic acid nitrile, 2-vinyl pyridine, maleicanhydride, methyl cinnamate, ethylene carbonate, halogenated ethylenecarbonate, bromobutyrolactone, methyl chloroformate, and sulfiteadditives, such as ethylene sulfite (ES), propylene sulfite (PS), andvinyl ethylene sulfite (VES). An additive of particular interest hereinis vinylene carbonate (VC) or a derivative thereof. In other particularembodiments, the additive is preferably selected from1,3-propanesultone, ethylene sulfite, propylene sulfite, fluoroethylenesulfite (FEC), α-bromo-γ-butyrolactone, methyl chloroformate, t-butylenecarbonate, 12-crown-4 ether, carbon dioxide (CO₂), sulfur dioxide (SO₂),sulfur trioxide (SO₃), acid anhydrides, reaction products of carbondisulfide and lithium, and polysulfide.

The electrolyte medium may or may not also include one or moresurfactants. The surfactants can be included to, for example, modify oradjust the electrolyte electron or ion transport properties. Thesurfactant can be either an anionic, cationic, or zwitterionicsurfactant.

Some examples of anionic surfactants include the fluorinated andnon-fluorinated carboxylates (e.g., perfluorooctanoates,perfluorodecanoates, perfluorotetradecanoates, octanoates, decanoates,tetradecanoates, fatty acid salts), the fluorinated and non-fluorinatedsulfonates (e.g., perfluorooctanesulfonates, perfluorodecanesulfonates,octanesulfonates, decanesulfonates, alkyl benzene sulfonate), and thefluorinated and non-fluorinated sulfate salts (e.g., dodecyl sulfates,lauryl sulfates, sodium lauryl ether sulfate, perfluorododecyl sulfate,and other alkyl and perfluoroalkyl sulfate salts).

The majority of cationic surfactants contain a positively chargednitrogen atom, such as found in the quaternary ammonium surfactants. Aparticular class of cationic surfactants considered herein include thequaternary ammonium surfactants. Some examples of quaternary ammoniumsurfactants include the alkyltrimethylammonium salts,dialkylmethylammonium salts, trialkylmethylammonium salts, andtetraalkylammonium salts, wherein the alkyl group typically possesses atleast 3, 4, 5, or 6 carbon atoms and up to 14, 16, 18, 20, 22, 24, or 26carbon atoms. Another group of cationic surfactants are the pyridiniumsurfactants, such as cetylpyridinium chloride. The counteranions in thecationic surfactants can be, for example, a halide, hydroxide,carboxylate, phosphate, nitrate, or other simple or complex anion.

Some examples of zwitterionic surfactants include the betaines (e.g.,dodecyl betaine, cocamidopropyl betaine) and the glycinates.

Examples have been set forth below for the purpose of illustration andto describe certain specific embodiments of the invention. However, thescope of this invention is not to be in any way limited by the examplesset forth herein.

Example 1 Preparation and Analysis of PAF-S, an S/N-DopedElectron-Conducting Polymer Electroactive Microporous Organic MolecularSieve Synthesis of PAF-S

The synthesis and characterization of the precursor, PAF, is disclosedin J. Mater. Chem., 2011, 21, 18208-18214. PAF-S was prepared bygrounding PAF powder (0.1 g) and sulfur (0.5 g) together, followed byheating at 155° C. for 12 h in a sealed vessel. The composite was thenheated to and kept at 300° C. for 1 hour under argon to evaporate thesurface sulfur.

Characterization of PAF-S

FIG. 1 shows the TGA curves of pure PAF and its sulfur composite PAF-S.The carbon yield of pure PAF-S at 700° C. was found to be 78%. Thesulfur content within the composite after heat treatment at 300° C. wasfound to be 30 wt %.

Composite Preparation and Electrode Assembly Using PAF-S

The sulfur electrode was made by casting a well-homogenized slurry ofPAF-S(65 wt %), carbon black (20 wt %), and PVdF (15 wt %) in NMP onaluminum foil using a doctor blade. The dried electrodes were pressedunder a hydraulic load of 1 ton for 1 minute before cutting into discsof 1.3 cm diameter. The discs were further dried at 60° C. under vacuumfor 24 hours before being transferred into a glovebox for batteryassembly. The batteries were assembled as 2032-type coin cells inside anargon-filled glovebox by using the PAF-S composite electrode as thecathode, lithium metal as the counter electrode, Celgard® 3401 as theseparator, and either 0.5 M LiTFSI/MPPY.TFSI or 1.0MLiPF₆/methylisopropylsulfone (MiPS) as the electrolyte.

Cell Performance of PAF-S Composite Electrode

FIGS. 2A, 2B and 3A, 3B show the cell performance for the PAF-Scomposite electrode at different temperatures using differentelectrolytes. FIG. 2A shows the discharge (Li uptake)/charge (Liremoval) curves of the PAF-S at a rate of 0.05 C (84 mA/g) in 1 MLiPF₆/MiPS at 25° C. FIG. 2B shows the cycle performance of PAF-Selectrode in 1 M LiPF₆/MiPS at 25° C. The PAF-S electrode exhibited areversible capacity of 1083 mAh g⁻¹ (second cycle) by using theelectrolyte of 1 M LiPF₆/MiPS. After 50 cycles, the capacity was stillmaintained at 630 mAh g⁻¹. FIG. 3A shows the discharge/charge curves ofthe PAF-S at a rate of 0.05 C in 0.5M LiTFSI/MPPY.TFSI at 50° C. FIG. 3Bshows the cycle performance of PAF-S electrode in 0.5M LiTFSI/MPPY.TFSIat 50° C.

Example 2 Preparation and Analysis of S/N-doped Mesoporous Carbon

Synthesis of S/N-Doped Mesoporous Carbon

Mesoporous carbon was synthesized by carbonization of a nanostructuredpolymeric composite that was obtained by self-assembly of a blockcopolymer (i.e., Pluronic® F127) and phenol resin (i.e.,phloroglucinol-formaldehyde) under acidic conditions via a soft-templatemethod (Advanced Materials, 2011, 23, 3450-3454). In a typicalsynthesis, phloroglucinol (26.2 g), F127 (EO₁₀₆—PO₇₀-EO₁₀₆, BASF, 52.4g), and aqueous HCl (10.0 g, 37 wt %) in ethanol (1.3 L, 200 proof) werecharged into a 2 L flask. The mixture was heated to reflux understirring before an aqueous formaldehyde solution (26.0 g, 37 wt %) wasadded. The reaction was continued for two hours and filtered. Yellowpolymer particles on the filter were washed with ethanol and dried in anoven at 120° C. for three hours. The carbonization was conducted undernitrogen gas flow in a tubular furnace by heating the polymer particlesto 850° C. at a heating rate of 2° C. per minute. The final temperature(850° C.) was held for two hours to ensure the complete decomposition ofthe template and carbonization of the polymer. Nitrogen-doped mesoporouscarbon was prepared by heat treatment of the above obtained mesoporouscarbon under NH₃ (50 mL/min) at 850° C. for two hours.

S/N-doped mesoporous carbon and sulfur composite was prepared bygrinding nitrogen-doped mesoporous carbon (0.1 g) and sulfur (0.3 g)together, followed by heating at 155° C. for 12 hours in a sealedvessel. The composite was then heated to and kept at 300° C. for 1 hourunder argon to evaporate the surface sulfur.

Characterization of S/N-Doped Mesoporous Carbon Material

FIG. 4 shows the TGA curve of S/N-doped mesoporous carbon, whosepreparation was described above. As shown, the sulfur loading within thecomposite after heat treatment at 300° C. was found to be 50 wt %.

Composite Preparation and Electrode Assembly Using S/N-Doped MesoporousCarbon

The sulfur electrode was made by casting a well-homogenized slurry ofS/N-doped mesoporous carbon (65 wt %), carbon black (20 wt %), and PVdF(15 wt %) in NMP on aluminum foil using a doctor blade. The driedelectrodes were pressed under a hydraulic load of 1 ton for 1 minutebefore cutting into discs of 1.3 cm diameter. The discs were furtherdried at 60° C. under vacuum for 24 hours before being transferred intoa glovebox for battery assembly. The batteries were assembled as2032-type coin cells inside an argon-filled glovebox by using theS/N-doped mesoporous carbon composite electrode as the cathode, lithiummetal as the counter electrode, Celgard® 3401 as the separator, and 0.5M LiTFSI/MPPY.TFSI as the electrolyte.

Cell Performance of S/N-Doped Mesoporous Carbon Composite Electrode

FIGS. 5A, 5B show charge/discharge profile and cycle performance for themesoporous carbon composite electrode using 0.5 M LiTFSI/MPPY.TFSI atroom temperature (typically, 18-22° C., or about 20° C.).

Example 3 Preparation and Analysis of S/N-Doped Mesoporous CarbonAdmixed with Carbon Nanotube (CNT), i.e., “CNT-Mesoporous Carbon”

Synthesis of CNT-Mesoporous Carbon

In this example, 10 wt % of carbon nanotube was added to nitrogen-dopedmesoporous carbon, and this used to prepare the sulfur composite.CNT-Mesoporous carbon was prepared via self-assembly of block copolymerand phloroglucinol-formaldehyde resin under acidic conditions. In atypical synthesis, a 2 L flask was charged with 26.2 g ofphloroglucinol, 52.4 g of Pluronic® F127, (EO₁₀₆—PO₇₀-EO₁₀₆, BASF), 10.0g of aqueous HCl (37 wt %) and 2 g of carbon nanotubes (CNT) in 1300 mLof ethanol (200 proof). The mixture was heated to reflux with stirring.To this solution, 26.0 g of formaldehyde aqueous solution (37 wt %) wasadded. The reaction mixture was stirred for two hours and then filtered.The obtained polymer particles were washed with ethanol and dried in anoven at 120° C. for 3 hours. Carbonization was conducted under anitrogen atmosphere at 400° C. for two hours at a heating rate of 1° C.per minute, which was followed by treatment at 850° C. for two hours ata heating rate of 5° C. per minute.

Characterization of CNT-Mesoporous Carbon Material

FIG. 6 shows the TGA curve for the CNT-mesoporous carbon material. TheTGA curve in FIG. 6 shows that the sulfur content within theCNT-mesoporous carbon material after heat treatment at 300° C. was 33 wt%.

Composite Preparation and Electrode Assembly Using CNT-Mesoporous CarbonMaterial

The sulfur electrode was made by casting a well-homogenized slurry ofCNT-mesoporous carbon material (65 wt %), carbon black (20 wt %), andPVdF (15 wt %) in NMP on aluminum foil using a doctor blade. The driedelectrodes were pressed under a hydraulic load of 1 ton for 1 minutebefore cutting into discs of 1.3 cm diameter. The discs were furtherdried at 60° C. under vacuum for 24 hours before being transferred intoa glovebox for battery assembly. The batteries were assembled as2032-type coin cells inside an argon-filled glovebox by using theCNT-mesoporous carbon composite electrode as the cathode, lithium metalas the counter electrode, Celgard® 3401 as the separator, and either 0.5M LiTFSI/MPPY.TFSI or 1.0M LiPF₆/methylisopropylsulfone (MiPS) as theelectrolyte.

Cell Performance of CNT-Mesoporous Carbon Composite Electrode

FIGS. 7A, 7B show the charge/discharge profile (FIG. 7A) and cycleperformance (FIG. 7B) of Li—S batteries based on CNT-mesoporous carboncomposite electrodes in 1.0 M LiPF₆/MiPS whereas FIGS. 8A, 8B showcharge/discharge profile (FIG. 8A) and cycle performance (FIG. 8B) ofLi—S batteries based on CNT-mesoporous carbon composite electrodes in0.5 M LiTFSI/MPPY.TFSI, at a current density of 84 mA g⁻¹ (20 wt %carbon) at room temperature. The composite electrode exhibited areversible capacity of 1316 mAh g⁻¹ (second cycle) by using theelectrolyte of 1 M LiPF₆/MiPS. After 50 cycles, the capacity was stillmaintained at 714 mAh g⁻¹. Using 0.5 M LiTFSI/MPPY.TFSI as electrolyte,the composite electrode delivered a reversible capacity of 1752 mAh g⁻¹(second cycle), and after 50 cycles, the capacity was still maintainedat 1126 mAh g⁻¹.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, those skilled in the art maymake various changes and modifications which remain within the scope ofthe invention defined by the appended claims.

What is claimed is:
 1. A composite useful as a cathode for alithium-sulfur battery, the composite comprising: (i) a mesoporouscarbon composition doped with nitrogen atoms and having elemental sulfurdispersed therein, wherein said mesoporous carbon composition containsmicropores and mesopores, and wherein said elemental sulfur is includedin an amount of 20-50 wt % by weight of the mesoporous carboncomposition, and said elemental sulfur substantially fills saidmicropores while substantially not filling said mesopores; (ii) aconductive carbon in an amount of at least 5 wt % by weight of thecomposite; and (iii) a fluoropolymer binder.
 2. The composite of claim1, wherein said conductive carbon is in an amount of at least 15 wt % byweight of the composite.
 3. The composite of claim 1, wherein saidconductive carbon comprises carbon black.
 4. The composite of claim 1,wherein said elemental sulfur is in an amount of 40-50 wt % by weight ofthe mesoporous carbon composition.
 5. The composite of claim 1, whereinthe fluoropolymer is comprised of polyvinylidene difluoride.
 6. Thecomposite of claim 1, wherein said conductive carbon comprises carbonnanotubes.
 7. The composite of claim 6, wherein said carbon nanotubesare in an amount of 5-20 wt % by weight of the mesoporous carboncomposition.
 8. The composite of claim 6, further comprising carbonblack.
 9. The composite of claim 8, wherein said carbon black is in anamount of 10-30 wt % by weight of the composite.
 10. The composite ofclaim 1, wherein said conductive carbon is in an amount of 5 wt % byweight of the composite and said binder is in an amount of about 10 wt %by weight of the composite.
 11. The composite of claim 1, wherein, incomponent (i), said elemental sulfur fills said micropores while notfilling said mesopores.